Abbreviation for “Human T-cell Lymphotropic Virus.” The two HTLV types, HTLV-I and HTLV-II are both retroviruses, and as a result, they are often confused with the most famous retrovirus, human immunodeficiency virus (HIV, originally named “HTLV-III” in early literature). Both HTLV types are readily transmitted from infected donors to blood recipients, but, in the vast majority of cases, neither causes actual clinical problems. HTLV-1 may be associated with two very scary-sounding diseases (which occur in only 2-4% of those infected at birth): Adult T-cell leukemia/lymphoma (ATL) and HTLV-associated myelopathy (HAM, also known as tropical spastic paraparesis). HTLV-II infections are not associated with ATL, but can occasionally lead to HAM.

HTLV-1 is endemic (widespread) in the Caribbean, parts of Japan, and parts of Africa and South America. HTLV-II, on the other hand, is increasingly seen in developed countries among IV drug users. The virus is transmitted sexually, through transfusion, and from mother to child (most often via breastfeeding). US blood centers are required to screen all donors for HTLV infection using an antibody detection test for both viruses, the anti-HTLV-I,II test.